Flag code analysis issues once per converter used (#21503)

Summary:
Fixes issue where the bot would leave multiple lines in the top review comment if, say, eslint found 30 issues, there would be 30 mentions of eslint having found issues.

See https://github.com/facebook/react-native/pull/21492 for an example of such a case, where `analysis-bot` left a spammy review at https://github.com/facebook/react-native/pull/21492#pullrequestreview-161837975.
Pull Request resolved: https://github.com/facebook/react-native/pull/21503

Differential Revision: D10219439

Pulled By: hramos

fbshipit-source-id: 75d32ef3bfeaa91ab614763a19494659ad1be0dd